Control equipment for stabilizing a platform mounted in a craft



Jan. 24, 1950 F. w. MEREDITH 2,495,591 CONTROL EQUIPMENT FOR STABILIZING A PLATFORM MOUNTED IN A. CRAFT Filed Feb. 21, 1944 IN1/e Nro/ 6.7 MMM, M f

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Patented Jan. 24v,Y 1950 UNITED STATES PATENT OFFICE (Clz- 3189-185),

5 Claims.

l(b). AY compass,

(c) A pitch detector or rate-of-pitch detector, (d). rA bomb sight,

(e) yA camera,

(fl A telescope.

The invention makes use of a motor' driving the platform about.` the stabilisation axis and a rate. of turn device, such as described in U; S.

Patent No. 2,455,939, the signal from: which controis the motor to stabilise the platform in a datum attitude. abouty the stabilisation axis.

Inevitably' the platform and/or the instrumentused to detectthe rate of turn thereof have inertia. and' if the latter` iscarried on the stabilised platform the elasticity of the mounting thereof or' of the coupling between it and' the motor together with said inertia provide a resonant system whereby, at frequencies closeto the resonant frequency,v large phase displacements between the movement of the motor and the movement of' the instrument occur whereby hunting is caused.` 'The object of this invention is to maintain theV required phase'y relationship' between the movement of the. motor and the signal so that the resulting acceleration ofthe platform ismaintained'. without requiring rigidity in the mounting which would resultk in undesirable vibration being transmitted from the aeroplane to the platform ybesides complicating the' problem of installation'r K i According. to this inventiony a pivoted platform is; associated with the rate-of-turn device a. signal: from; which controls the operation of a motor .operatively connected with` the platform through ak gear boxf; the'platform, gear box and motor are mounted on a spring mounting and the direction of rotation of' the. motor is arranged to bein the opposite sense to the direction of rotation of they platform` and the masses are so selected that the product off the gear ratio and the-'moment of inertia of the motor is not less than the moment of inertia of the platform.

Preferably' the operative connection between the 5'5 motor and the platform is made as rigid as possible.

It wil-l be understood that the inertial reaction due to acceleration of' the platform is counterbalanced by the opposing inertial reaction due to acceleration of the motor 'so that there isl no resulting deflection of the spring` mounting, or, if the product of the gear' ratioand the` inertia of the motor exceeds the inertia of they platform (and any masses moving therewith) the. resulting deflection of the spring mounting is in the oppositesense to that which would cause` hunting. Further, it will be appreciated. that other periods due to inevitable elasticity of parts intended to be rigid will be shorter than. the period provided by the spring mounting and will thereforeV be absorbed by' deflection of the spring mounting. Vibration of the aeroplane will likewise: bev absorbed as in a conventional antirvibration mounting.

In the accompanying drawings:

Figure 1 represents diagrammatically onev embodiment for stabilising a platform pvote'd in aircraft about an axis parallel to the aircraft pitchh axis and,

Figure Z'another embodiment of the invention. Referring to Figure l, the platform II is car:- riedon the output shaft I5 of the gear-box' I3 which is mounted in` the framework of the aircraft by springs I4. The shaft I5 is parallel to the pitch axis of the aircraft.

The rotor of the two phase electric motor l2, carried in gear-box I3, is coupled to the` input of gear-box I3 as shown. The axis of the rotor is parallel to shaft l5 and, as shown, the gearing is such that the` rotor' and the shaft I5 rotate in opposite senses.

A device i6 sensitive to rate of turn about the pitch axis of thev craft is mounted on platform |I. rThis device is of the type described inA U. S. Patent No. 2,455,939 and gives two A C. outputs; the first output,`appearing at terminals I'l is of constant amplitude and is derived from the output of the device 218, entitled Automatic volume control of Figure 5B of Patent specification No. 2,455,939, while the. second' output, appearing at terminals I8, has an amplitude proportional to the rate-of-pitch to which the device I6 is subjected and is derived from the output of the Y-Y amplifier 216 of Figure 5B of the aforementioned patent specification. The two outputs have .the same frequency and are in phase or in antiphase as the sense of the rateof-turn. is in one direction or thel other. y 1

The .terminals I Sfaref connected to the'wndlng,

I9 of motor I2 while terminals I1 are connected through the phase-shifting device 2I to the other winding 20 of motor I2. The phase of the signal from terminals I'I is shifted through 90 by device 2I so that the signals applied to windings I9, 20 are in quadrature.

The motor I2 is of the type in which the acceleration is linearly related to the amplitude of the signal applied to signal winding I9. Such motors are very well known and are exemplified by two-phase hysteresis motors. A convenient type of motor is that described in British Patent No. 576,248 filed Dec. 31, 1943, and Jan. 27, 1944, and No. 576,249 filed Dec. 31, 1943, and Sept. 8, 1944, said patents corresponding to my U. S. patent applications Serial Nos. 526,435 and 526,436 respectively. The acceleration of motor I2 is therefore proportional to the amplitude of the signal from terminals I8 and hence to the detected rate-of-pitch. The platform II rotates in the opposite sense to the detected rate-ofturn and is thus stabilised in a substantially constant position: as soon as the craft turns about the pitch axis, the device IS emits a signal which operates motor I2 to cause the platform II to rotate at such a rate that the device I6 is subject to zero rate of pitch i. e. the platform is rotated at a rate which is equal and opposite to the detected rate of pitch of the craft. If the two rates of turn are not equal and opposite, the device I6 emits a signal altering the rate of turn of the platform until the two rates of turn are again equal and opposite.

Unless precautions are taken, the stabilising system described above is liable to hunt. If R is the gear ratio of gear-box I3, Im is the moment of inertia of the motor I2 and Ip is the moment of inertia of the platform II, the gearbox I3, motor I2 and platform II are so designed that RIm is not less than Ip. It can be shown mathematically that this is the only condition to ne fulfilled to render the system stable and hunt-free.

Figure 2 illustrates an alternate arrangement to that shown in Figure 1. In this case, the casing of the gear-box i3 is rigidly attached to the platform ii and the output shaft I5 of the gear-box is mounted eiastically in the aircraft. r'he axis of the rotor motor I2 and the output shaft i5 are again parallel but, as shown, the gearing is such that the rotor and output shaft rotate in the same sense. In this embodiment of the invention, the moment of inertia of the platform ii includes that o the gear-box I3, in which case the components are again designed so that RIm is not less than Ip. It can be shown mathematically that if this condition is fulfilled the system is stable and hunt-free.

It will be understood that if the acceleration of the motor is proportional to the detected rateof-turn, this is `equivalent to the motor having a speed proportional to the displacement of the platform II or the integral of the rate-oi-turn thereof.

I claim:

l. Control equipment for stabilising aplatforrn pivoted in a craft about an axis about which it is to be stabilized comprising, a motor having a rotor axis arranged parallel to said platform a gear bez; coupling the motor to the platform to eifect rotation thereof about the said in the opposite direction to the motor, a rate-ofturn device mounted on the platform and responsive togthe rotation thereof about the said axis, control connections between the device and sil) the motor, said control connections operating the motor in a direction to eiect stabilization of the platform and at a rate proportional to the displacement of the platform about said axis, and a spring mounting carrying platform, gear box and motor, the product of the gear ratio and the moment of inertia of the motor being not less than the moment of inertia of the platform and parts mounted thereon.

2. Control equipment for stabilizing a platform pivoted in a craft about an axis about which it is to be stabilized comprising a gear box having parallel input and output shafts adapted to rotate in opposite directions the platform being mounted on the output shaft, a motor carried by the gear box and coupled to the input shaft, a spring mounting for the gear box. a rate-of-turn device mounted on the platform and responsive to the rotation thereof about the axis of the output shaft and control connections between the device and the motor, said control connections operating the motor in a direction to effect stabilization of the platform about the said axis and at a rate proportional to the displacement of the platform about the said axis, the product of the gear ratio and the moment of inertia of the motor being not less than the moment of inertia of the platform and the device mounted thereon.

3. Control equipment for stabilizing a platform pivoted in a craft about an axis about which it is to be stabilized comprising a gear box having a casing and parallel input and output shafts adapted to rotate in the same direction the platform being secured to the gear-box casing, a motor carried by the gear box and coupled to the input shaft, a spring mounting resisting rotation of the output shaft, a rate-of-turn device mounted on the platform and responsive to the rotation thereof about the axis of the output shaft and control connections between the device and the motor, said control connections operating the motor in a direction to eect stabilization of the platform about the said axis, the

; product of the gear ratio and the moment of inertia of the motor being not less than the moment of inertia of the platform, the device and the gear box casing.

4. Control equipment for stabilizing a platform pivoted in a craft about an axis about which it is to be stabilized comprising, an electric motor having a rotor axis arranged parallel to said platform axis, a gear box coupling the motor to the platform to effect rotation thereof about tne said axis in the opposite direction to the motor, a rateofturn device mounted on the platform and responsive to the rotation thereof about the said axis, electrical connections between the device and the motor, said electrical connections operating the motor in a direction to effect stabilization of the platform and at a rate proportional to the displacement of the platform about the said axis, and a spring mounting carrying platform, gear box and motor, the product of the gear ratio and the moment of inertia of the motor being not less than the moment of inertia of the platform and parts mounted thereon.

5. Control equipment for stabilizing a platform pivoted in a craft about an axis about which it is to be stabilized comprising, a motor having a rotor axis arranged parallel to said platform axis, a gear box coupling the motor to the platform to effect rotation thereof about the said platform axis in the opposite direction to the motor, a device mounted on the platform and generating a signal proportional to the rate of turn of the platform about the said axis, control connections for applying the signal to accelerate the motor in a direction to stabilize the platform and at a rate proportional to that of the signal, and a spring mounting carrying platform, gear box and motor, the product of the `gear ratio and the moment of inertia of the motor being not less than the moment of inertia of the platform and parts mounted thereon.

FREDERICK WILLIAM MEREDITH.

REFERENCES CITED The following references are of record in the tile of this patent:

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